Investigators in eight interacting laboratories are analysing changes in neuronal organization in various mammalian species including man. Evidence of plasticity is found after four different kinds of treatment: 1) Sensory deprivation early in life results in altered electrical responses of single neurons in kittens, and can cause sensory acuity losses in man, apparently due to a selective neuronal atrophy of disuse or a selective retardation of neuronal development. 2) Motor or sensorimotor deprivation early in life, studied in kittens, can result in drastically subnormal development of visually guided movements and of the perception of spatial arrangements. 3) Early brain lesions, if inflicted before a critical age, can result in the development of abnormal neuroanatomical connections in visual pathways, with adaptive or maladaptive functional consequences judged by electrical activity of single cells, and by actual behavior. Some abnormal connections produce no noticeable behavioral effect unless "unmasked" by a later brain lesion. 4) Lesions of the vestibular apparatus of adolescent monkeys cause a loss of eye-head movement control which gradually recovers; the recovery is due to specific changes in sensorimotor functions which have been linked to part of the cerebellum. BIBLIOGRAPHIC REFERENCES: Held, R. Early deprivation and meridional variation in visual acuity. In: NRP Bulletin, "Perceptual and Neuronal Aspects of the Visual System," J. Dowling, R. Held, and E. Poppel (Eds.). Boston: Neurosciences Research Program, 1977, in press. Held, R., Mohindra, I., Gwiazda, J. and Brill, S. Visual acuity of astigmatic infants and its meridional variation. (Abstract) Association for Research in Vision and Opthalmology (ARVO), Sarasota, 1977.